1. Field of the Invention
This invention relates to a lithographic process for exposing a photosensitive material to irradiation with a radiation beam through a masking material support film provided with a masking material pattern-wise, and also to a lithographic mask structure.
2. Related Background Art
X-ray lithography has many distinguished advantages over the conventional lithography using visible light or ultraviolet light owing to the properties proper to the X-rays such as rectilinear propagation, non-interference, low diffraction, etc., and thus has been regarded as an important and useful means for submicron lithography or quater-micron lithography. However, in spite of the many advantages over the conventional lithography using visible light or ultraviolet light, the X-ray lithography still has such a disadvantage as a low productivity that is, a high cost, owing to the power shortage of X-ray source, the low sensitivity of resist, the difficult alignment, the difficult choice of masking material, the difficult processing procedure, etc. and thus its practical application has been considerably delayed.
A most successful practical resist among those so far disclosed in the literature, etc. on the lithography is polymethyl methacrylate (PMMA), which is also a successful resist for electron beams, which can particularly meet the requirement for less than 0.1 .mu.m, and thus no other competitive materials have been found yet. A still remaining important problem of X-ray lithography is a problem of sensitivity. It is said that the X-ray utilization efficiency of resist in the X-ray lithography is usually 0.3% at most, which is a cause for lowering the sensitivity. For example, in the case of PMMA using a Pd K.sub..alpha. beam as X-ray, it is said that the sensitivity is 1,000 to 2,000 mJ/cm.sup.2, where the sensitivity is given by an X-ray irradiation dosage at the exposure. That is, the lower the irradiation dosage, the higher the sensitivity. A practical resist of high sensitivity is chloromethylated polystyrene (CMS), which has a sensitivity of about 100 mJ/cm.sup.2 under the same conditions as above. A condition for practical application of X-ray lithography is a necessary provision of X-ray resist having a sensitivity of not more than 10 mJ/cm.sup.2, and its development has been desired.
Now, researches have been made for higher productivity in three directions, i.e. intensity of beam source, X-ray transmission of masking material support body and X-ray intransmission of masking material, and sensitivity of resist, but no rapid and remarkable development is expectable owing to many restricting factors. However, a higher sensitivity of resist can be an essential condition from a fact that a device would be damaged if the X-ray is too strong. It is needless to say that this condition is necessary and essential even if the properties of mask and beam source are improved in the future.
Description will be made of X-ray lithographic mask below. In the lithography using visible light and ultraviolet light, a glass plate and a quartz plate are used as a masking material support body (light-transmissible body). However, the wavelength utilizable in the X-ray lithography is, for example, 1 to 200 .ANG.. The so far available glass plate or quartz plate has a large absorption in the X-ray wavelength region, and must be as thick as 1-2 mm to maintain flatness, with a failure to transmit the X-ray therethrough. Thus, the glass plate or quartz plate is not suitable as a material for X-ray lithographic masking material support body.
Generally, the X-ray transmissivity depends on the density of a material, and thus inorganic or organic materials of low density have been studied as materials for an X-ray lithographic masking material support body, and include, for example, such inorganic materials as simple substances, e.g. beryllium (Be), titanium (Ti), silicon (Si), and boron (B), their compounds, etc. and organic materials such as polyimide, polyamide, polyester, poly-p-xylylene (trade name parylene) produced by Union Carbide, Co.,), etc.
In an actual application of these materials as a material for X-ray lithographic masking material support body, it is necessary to make them into a thin film to make the X-ray transmission as high as possible. It is required that the film thickness is not more than a few .mu.m for the inorganic materials and not more than a few tens .mu.m for the organic materials. In the formation of a masking material support body (which will be hereinafter referred to as "a masking material support film") composed of, for example, an inorganic thin film or its composite film to this end, there has been proposed a process which comprises forming a film of silicon nitride, silicon oxide, boron nitride or silicon carbide on a silicon plate of good flatness by vapor deposition, etc., and then removing the silicon plate by etching.
On the other hand, the X-ray lithographic masking material (X-ray absorbing material, which will be hereinafter referred to as "a masking material") support on the above-mentioned masking material support film includes films of materials generally having a high density, for example, gold, platinum, tungsten, tantalum, copper, or nickel, preferably films having a thickness of 0.5 to 1 .mu.m. Such a masking material can be formed, for example, by forming a thin film of high density masking material throughout on a masking material support film, then applying a resist thereto, depicting a desired pattern on the resist by an electron beam, light, etc., and then forming the desired pattern by etching, or other means.
In the conventional X-ray lithography as mentioned above, the masking material support film has a low X-ray transmissivity and thus must be made considerably thin to obtain a sufficient X-ray transmission. It has been a problem to make such a thin film.